And if his defense is that good, he is silly to not add.
Castillo has averaged 1 DRS for every 44innings and cost $36mill for 3 years. Jarrod Dyson and his career OPS of .677 has averaged 1DRS for every 58.5 innings and was projected to earn $36mill over 3 years, although that does look less likely right now.
Right now Castillo is a costly AAA player with potentially game-changing defensive skills. But in the majors, he might also become solid trade bait. Or make Bradley solid trade bait. In fact, if I was a GM who needed a CF, I'd be trying to fleece Boston out of this guy with a bad contract I already had...